1. Field of the Invention
The present invention generally relates to transducers for measuring fluid pressure forces, and more specifically such transducers which employ resonant elements to sense the force.
2. General Background Considerations
In force measuring methods, including fluid pressure force measurements, systems generally depend upon precise mechanical fabrication of either the entire system of mechanical ratio arms of a mass balance or force beam balance, or alternatively upon precise spring or compliant elements. In the balance schemes, knife edges and jewel bearings and gears are often used in accurate systems. All suffer from the need for precise mechanical parts which tend to wear, age from mechanical stress and suffer further from the finite size limits of the smallest increment of force that can be reliably measured. For even moderately accurate requirements of perhaps somewhat better than one percent, devices that cope with these factors cannot at present be made inexpensively. The complexity and precision of mechanical parts for systems of better than one part in a thousand accuracy become very costly. For very accurate weighings, for instance, as used in laboratory chemical measurements, the methods used usually employ a mass balance scheme. The use of the balance method allows the weighing masses to be transported easily to another location allowing precise calibration against other accurate standards. The local values of gravitational force do not therefore affect the measurement. One makes a ratio measurement by means of a precise mechanically stable beam of known ratio. For accurate work, however, the weights must be handled very carefully so as not to wear or remove any material and must be kept scrupulously clean and free of surface adsorbed vapors or corrosion products. For very accurate weight measurements in laboratory work, the local value of temperature and barometric pressure will affect the buoyancy of the weights in the air and the bending and distortion of the beam by the forces involved all must be taken into account.
In the spring or compliance element systems, one relies upon the stability of the elasticity of a material used to fabricate a spring or diaphragm element and suffers equally from the need for very accurate dimensional and shape fabrication. The compliance or elasticity of the materials employed depend upon use history, work-hardening, environmental exposure, corrosion and creep under steady load. All materials of this type show some deformation or creep, especially if a large relative strain is employed in order to produce a significant magnitude of response to the force being measured. Additionally, compliant element systems suffer from hysteresis where forces vary rapidly and frequently. Spring or compliant systems suffer additionally from inaccuracies in the measurement of the deformation of the strained or compliant element. Electrical strain gauges are often used to make measurements of the deformation in a compliant system. The use of strain gauge electrical output systems for observing the strain or deformation introduces additional problems associated with the degree of coupling of the strain to the measured effect. A concept in evaluating such measuring schemes has evolved to represent the variation in the measured parameter as a function of the magnitude of the value of the observed quality. In a resistance wire strain gauge, this gauge factor represents the percent change in resistance per percent change in strain. The advantage that resistance wire strain gauges offer is that the gauge factor of many schemes is greater than one in that the resistance varies by a larger magnitude than the degree of strain. Gauge factor in such systems therefore represents a kind of magnification of the basic strain of the compliant element into the coupled parameter measured. The variation in the magnitude of the observed parameter is nonetheless a very small portion of the magnitude of that parameter since strains of acceptable hysteresis or creep rarely can exceed but a small fraction of a percent of the volume or dimension of the compliant element. The yield or permanent set point of most materials is defined by convention as the magnitude of stress that will produce one percent permanent strain. It is inherent therefore that the volume or dimension of the stressed compliant elements cannot be deformed very greatly or large inaccuracies will develop. Springs, diaphragms and such like compliant elements are therefore rarely strained any more than one part in a thousand in order to provide some degree of stability. This makes clear the value and the necessity of high stability in the measurement of the coupled parameter in strain gauge and similar indirect measuring systems. The gauge factor concept may be modified to include the stability factors found in the various kinds of indicator systems. When one examines mass balance schemes, strain gauges, compliant elements and related systems from the standpoint of a merit factor, the relative insensitivity of balance schemes is to a large degree offset by the higher order of accuracy and reproducibility obtained. The strain gauge systems lie in an area below this. It is common that in fluid pressure force systems the calibration of precision systems is made by employing a precision piston and known forces obtained from weights or balance systems. The accuracy and stability of the compliant systems is classically, therefore, well below that of the highest accuracy force measuring systems. In recent years, the integration of a compliant diaphragm element and resistance strain gauge into a monolithic silicon semiconductor system has resulted in diaphragm strain gauge systems of notable sensitivity and potentially improved accuracy. Schemes of this type, however, from the standpoint of overall merit factor appear to be only slightly better than classical strain gauges.
There also exists a number of force transducing sensors that employ resonant element structures in an effort to increase the sensitivity and accuracy of the coupled measured parameter to the force measuring system. All such schemes generally represent a similar integration of the compliant element with a size or shape determined oscillating system in an effort to accurately measure the deformation of the compliant element. All previous schemes suffer from complexities in coupling the strain or force to be measured to the resonating element and although increased resolution is often obtained the stability has not generally been improved substantially. There exists some schemes of force coupling technique that have permitted the most accurate high resolution force measurements to be made using the mechanical resonator as a sensor. These schemes have generally been very costly and their merit factor, though substantially greater than those previously discussed, still lies significantly below the region sought after in the subject disclosure.
Compliant elements are generally fabricated of what are known as polycrystalline materials in contradistinction to single crystal materials. An aspect of the noteworthy advantages of the silicon strain gauge-silicon diaphragm integrated sensors is the utilization of highly perfect single crystal material for both strained diaphragm and resistance sensor element. In recent years it has become possible to fabricate large volumes of highly perfect single crystal substance, thereby enabling the elastic properties of such materials to display extraordinary stability and uniformity due to the vastly lower defect density in the solid structure. Most normal materials, metals, plastics, ceramics and other substances are in nature polycrystalline and rely for their elastic properties upon interparticular cementing and attaching forces, as well as upon the more fundamental interatomic forces of the single crystal substance. Such materials vary greatly from batch to batch and as a product of their fabrication history. A noteworthy aspect of the single crystal substance is the product-to-product uniformity and the homogeneity of the various dispersed regions of a shape fabricated from such substance.
A survey of contemporary limits of force measurements is contained in the National Bureau of Standards Special Publication 343, "Precision Measurement and Fundamental Constants", Proceedings of the International Conference Held at the National Bureau of Standards, Gaithersburg, Maryland, Aug. 3-7, 1970. See, for example, the sections in this work relating to length standards and the velocity of light, electrical standards, faraday constant and atomic masses, Josephson affect and gravitational constants.